But the authors and an accompanying comment caution that there remains controversy as to whether folic acid supplementation can lead to improved outcomes for other cardiovascular conditions.
Professor Xiaobin Wang, Children’s Memorial Hospital and Children’s Memorial Research Center, Northwestern University Feinberg School of Medicine, Chicago, USA, and colleagues did a meta-analysis (a study combining previous trials) of eight trials of folic acid that had stroke reported as one of the endpoints.
Folic acid supplementation lowers the concentrations homocysteine in the blood. High amounts of homocysteine in the blood are thought to increase the risk of stroke, as well as that of cardiovascular disease and deep vein thrombosis.
They found folic acid supplementation reduced the relative risk of stroke by an average of 18 per cent. In subgroup analyses, an even greater reduction of risk was seen when the treatment lasted over 36 months (29% less risk); if it reduced the concentration of homocysteine in the blood by more than 20% (23% less risk); or if the patient had no previous history of stroke (25% less risk).
The study also found that in areas that did not already have supplementation through fortified or partly fortified grain, folic acid supplementation decreased the risk of stroke by 25%.
The authors conclude: "Our meta-analysis provides coherent evidence that folic acid supplementation can significantly reduce the risk of stroke in primary prevention."
But they caution: "To efficiently assess the efficacy and causality of folic acid supplementation on stroke, future clinical trials should be done in regions without grain fortification, with a longer period of follow-up (4 years or longer), and among individuals without a history of stroke.
"The issue of folic acid supplementation alone versus folic acid in combination with other B vitamins, as well as optimum dosage, should also be carefully considered in future trials."
In the accompanying comment, Dr Cynthia Carlsson, University of Wisconsin School of Medicine and Public Health, Wisconsin, USA, says: "Although this meta-analysis helps clarify answers to some questions about the role of homocysteine lowering in CVD prevention, ongoing randomised trials are needed before we can conclude that the benefit of continued use of previously deemed 'safe' vitamin supplements outweighs the risk of other adverse CVD outcomes.”
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